Different techniques have already been proposed to quantitatively determine the absorption and reduced scattering coefficients of turbid media. See Welch, A. J.; van Gemert, M. J. C., Optical Thermal Response of Laser Irradiated Tissue; Plenum Publishing Corp., New York, 1995, and references therein. Most of the non-invasive methods are based on the measurement of spatially and/or temporally-resolved reflectance. The principle is as follows: the turbid medium is illuminated by a collimated or focused light source. The backscattered light is measured by one or several detectors. Different types of measurements are possible, depending on the time-dependence of the illuminating source: steady-state (continuous source), time-domain (short pulsed source) or frequency domain (amplitude modulated source). The present invention relates to the case of steady-state measurements, performed at different distances ρ between the source and the detectors. However, the technique presented here can be complemented by time- or frequency-domain measurements.
The range of ρ values is an important point to consider, when comparing different methods based on the measurement of the reflectance. First, the probed volume of the turbid medium is related to the source-detector separation ρ. The larger the source-detector separation, the deeper the average depth probed. Second, depending on the range of ρ, different mathematical processing must be used to obtain the optical properties from the raw data.
At least two cases must be distinguished.
1) The first case corresponds to source-detector separations larger than several transport mean free paths. For typical biological tissue optical properties (W. -F. Cheong, S. A. Prahl, and A. J. Welch, “A Review of the Optical Properties of Biological Tissues,” IEEE J. Quantum Electron. 26, 2166-2185 (1990)), this case corresponds to a source-detector separation larger than 2 mm. An analytical form of the reflectance can be obtained from the diffusion equation, if the absorption coefficient μa is sufficiently lower than the scattering coefficient μs (typically ten times). In such a case, the relevant optical properties are the refractive index, the absorption coefficient and the reduced scattering coefficient. The average depth of probing is on the same order as the source-detector separation ρ.
Such methods have been already published, and are the object of patents (Ref., U.S. Pat. No. 5,517,987 Tsuchiya, U.S. Pat. No. 5,676,142 Miwa et al.).
2) The second case corresponds to source-detector separations close to one transport mean free path. For biological tissues2, such source-detector separations correspond typically to distances from 0.1 to 2 mm. The average depth probed is on the order of 1 mm. Such small source-detector separations enable the measurement of the optical properties of a small tissue volume.
Wang et al. (U.S. Pat. No. 5,630,423) proposed a method for the determination of the reduced scattering coefficient only, using an optical beam of oblique incidence. Moreover, their analysis does not include the effect of the phase function. Kessler et al. (U.S. Pat. Nos. 5,284,137 and 5,645,061) proposed a method for measuring the local dye concentration and scattering parameters in animal and human tissues, based on spatial and spectral measurements. However, their methods do not enable the simultaneous determination of the absorption coefficient, reduced scattering coefficient. Other publications may be of concern:                1. See “Welch, A. J.; van Gemert, M. J. C. Optical Thermal Response of Laser Irradiated Tissue; Plenum publishing Corp., New York, 1995”, and references therein.        2. W. -F. Cheong, S. A. Prahl, and A. J. Welch, “A Review of the Optical Properties of Biological Tissues,” IEEE J. Quantum Electron. 26, 2166-2185 (1990).        